Using a membrane to separate components of fluid mixtures is well-developed technology of great commercial significance. Separations of commercial importance include oxygen/nitrogen, carbon dioxide/nitrogen, carbon dioxide/methane and water from alcohols. In general, membrane separation processes involve bringing a fluid mixture in contact with one side of a permeable membrane. The composition of the membrane is chosen so that the components of the mixture permeate through the membrane at different rates. The preferentially permeable component(s) permeate faster than the less permeable component(s) thereby effecting a separation of the components. The commercial utility of a gas separation membrane depends on both the rate at which the most permeable component passes through the membrane and on the relative rates of permeation of the given components of the mixtures, referred to as selectivity of the membrane.
It is clearly desirable to maximize both the permeation rate and selectivity so as to achieve the optimum separation of the components through a given membrane area and at given pressure. It is well recognized, however, that there tends to be a tradeoff between permeability and selectivity, that is, highly permeable membranes tend to have low selectivity and vice versa. This situation is described by the Robeson plot of selectivity vs. permeability of diverse gas mixtures that is well known in the field of membrane technology. In addition to permeability and selectivity, the ideal membrane should possess other desirable properties, such as high chemical stability to the components of the gas mixture, including minor components which may be present as contaminants. It should be thermally stable at its upper use temperature, be mechanically robust, and easy to fabricate into thin films.
Among the separations of substantial industrial interest is that of the components present in air. Air is a gas mixture comprising about 21 mol % oxygen, about 78 mol % nitrogen and small amounts of other components. The separation of air to provide oxygen enriched air (OEA) and/or nitrogen enriched air (NEA) is commercially significant. For example, NEA may be used to provide an inert gas composition in tanks carrying highly combustible liquid fuels. OEA may be fed to internal combustion engines to improve engine efficiency or used to treat patients having respiratory difficulties, for example.
U.S. Pat. No. 6,478,852 discloses nonporous polymeric membranes useful for producing nitrogen enriched air. Certain amorphous copolymers of perfluoro-2,2-dimethyl-1,3 dioxole (PDD) are particularily preferred because they have a unique combination of superior permeability and selectivity for a variety of gas mixtures. In some preferred embodiments, the copolymer is copolymerized PDD and at least one monomer of perfluorinated or partially fluorinated compounds selected from the group consisting of tetrafluoroethylene (TFE), perfluoromethylvinyl ether, vinylidene fluoride, hexafluoropropylene and chlorotrifluoroethylene. Copolymers of PDD and such comonomers are disclosed as nonporous gas permeable membranes in U.S. Pat. Nos. 5,914,154, 5,960,777 and 6,126,721.
U.S. Pat. No. 6,126,721 discloses a process to obtain oxygen enriched air using a membrane separation module. A particularly useful membrane structure employs a substrate of small diameter, microporous hollow fibers coated with a very thin layer of perfluoro-2,2-dimethyl-1,3 dioxole (PDD)/tetrafluoroethylene copolymer. In some preferred embodiments, the copolymer is copolymerized PDD and at least one monomer selected from the group consisting of tetrafluoroethylene, perfluoromethylvinyl ether, vinylidene fluoride and chlorotrifluoroethylene. In particular, the PDD/TFE copolymer is sold under the trademeark Teflon® AF (more fully described herein, below) and shows a highly desirable combination of properties as a membrane material, but even better membrane performance than that obtained from these PDD/TFE dipolymers is desirable.
U.S. Pat. Appl. No. 2005/0228152 discloses anti-reflective coatings comprising a fluorinated amorphous copolymer wherein said copolymer contains at least one functionalized repeating unit. Among the copolymers identified for this application is a copolymer of PDD and vinyl acetate.
U.S. Pat. No. 4,439,217 discloses polymers of vinyl pivalate as permselective elements for gas separations. The polymer may also contain olefinic compound comonomers. Among the olefinic comonomers cited are fluorinated olefins such as vinyl fluoride, vinylidene fluoride, tetrafluorotheylene, chlorotrifluoroethylene, and hexafluoropropylene. No cyclic or cyclizable perfluorinated comonomers are disclosed.
U.S. provisional application 62/174,936 discloses partially fluorinated polymeric membranes for separating mixed gases such as oxygen and nitrogen mixtures. Those membrane polymers are produced by copolymerization of monomers including a perfluorinated cyclic or cyclizable compound and an alkylvinyl ester of structure H2C═CHOC(O)R in which R is a linear or branched alkyl group of from 1 to 5 carbon atoms.
There is a continuing need for selectively permeable membranes that provide more effective separation of selected gas mixtures, such as oxygen and nitrogen in air, than are known in the art. It is especially desirable that the membrane have a sufficiently high glass transition temperature to allow separation operations at higher temperatures.